U.S. Pat. No. 5,478,499 discloses a blue emitting phosphor that can be used in a vacuum fluorescent display, a field emission display, a write head of a printer, a back light for a liquid crystal display, etc. The phosphor of the '499 patent is capable of emitting a blue luminous color at a voltage of 100V or less. The phosphor is a ZnO--Ga.sub.2 O.sub.3 matrix doped with Li (lithium) and P (phosphorus).
FIG. 2 shows a flow chart of a method for preparing the above blue emitting phosphor of the '499 patent. First, ZnO, Ga.sub.2 O.sub.3, and Li.sub.3 PO.sub.4 are mixed. Next, the resulting mixture is subjected to a first sintering step for approximately 3 hours at a temperature of 1200.degree. C. and in a normal air atmosphere. The matrix is then milled, preferably with a ball mill, such that lumped matrix particles are evenly dispersed. To remove excess Li.sub.3 PO.sub.4, the dispersed matrix particles are washed with nitric acid. Subsequently, the washed matrix is subjected to a second sintering step for 1 to 3 hours at a temperature of 1100.degree. C. under a reducing atmosphere. Subsequently, the matrix is classified using a sieve, thereby obtaining the phosphor. The phosphor has an x value of 0.08 to 0.20 and a y value of 0.05 to 0.25 in a CIE chromaticity diagram. However, the blue emitting phosphor produced as described above has a low brightness, i.e., a low light emitting efficiency.